Effects of Nonstoichiometry and Cocatalyst Loading on the Photocatalytic Hydrogen Production with (Y1.5Bi0.5)1−xTi2O7−3x and (YBi)1−xTi2O7−3x Pyrochlores

By adjusting the Ti/(Y+Bi) ratios during synthesis, nonstoichiometric pyrochlores of (Y_1.5Bi_0.5)_{1 ? x}Ti₂O_{7 ? 3x} and (YBi)_{1 ? x}Ti₂O_{7 ? 3x} were prepared by an aqueous sol–gel method and annealed at different temperatures. The materials were characterized by X‐ray diffraction and UV–vis reflectance spectroscopy. The samples were tested for photocatalytic hydrogen production in the presence of methanol as sacrificial agent after being loaded with nanoparticles of rhodium or platinum acting as cocatalysts. It was found that materials being completely inactive in a stoichiometric composition can be tuned to good photocatalysts by optimizing the Ti/(Y+Bi) ratio. The increase in activity is supposed to derive from an optimization of the TiO₆‐octahedral geometry due to the generation of vacancies inside the structure. Increasing the Bi loading shifts the absorption edge into the visible, but unfortunately, an increase of the bismuth content in the structure also leads to stability issues during photocatalysis, which can be suppressed or at least weakened by a higher cocatalyst loading.     

Structural and Mössbauer Investigation of Nanocrystalline SrFe1−xTixO3−δ

Crystal structure and cation distribution of nanocrystalline SrFe_1?xTi_xO_3?δ (0 ≤ x ≤ 0.3) synthesized by combined high‐energy ball milling and solid‐state reactions are investigated using Neutron powder diffraction and Mössbauer spectroscopy. Ti doping stabilizes the single phase tetragonal structure with I4/mmm space group up to x = 0.3. The neutron and Mössbauer data confirm that Fe exists in three different sites both crystallographically as well as magnetically in all the four compositions. The cation distribution at various sites is established through Rietveld refinement.     

Preparation of Sr1−xCaxLiAl3N4:Eu2+ Solid Solutions and Their Photoluminescence Properties

A series of quaternary nitride solid solutions with a general formula of Sr_1?xCa_xLiAl₃N₄:0.5%Eu²⁺ was synthesized by a solid‐state reaction method. The experimental results showed that a proper amount of Ca‐doping can improve the crystallinity and the photoluminescence properties of the produced phosphors. Rietveld refinement showed that the volume of the unit cell shrank with the increase of Ca substitution for Sr, which resulted in a red shift of the emission spectra from 654 to 665 nm under blue excitation at 475 nm. Rietveld refinement and CASTPE calculations suggested that Ca²⁺ ions prefer to occupy the smaller Sr(I) sites in the crystal lattice, which increases the amount of Eu²⁺ ions in Sr(II) sites and enables the tuning of the chromaticity coordinates of the obtained phosphors. The thermal stability of the produced phosphors is better than that of commercial Sr₂Si₅N₈:Eu²⁺ phosphor. The experimental results qualify the solid‐solution Sr_1?xCa_xLiAl₃N₄:0.5%Eu²⁺ for consideration as a potential candidate for application in white LEDs.     

Microwave Dielectric Properties of Aluminum‐Substituted Ba6−3xNd8+2xTi18O54 Ceramics

According to solid‐state reaction routine, microwave dielectric ceramics of aluminum‐supplanted Ba_6?3xNd_8+2xTi₁₈O₅₄ (0.5 ≤ x ≤ 0.75) ceramics were synthesized and the effects of composition on microwave dielectric properties were determined. As x value increasing from 0.5 to 0.75, with high‐quality factor values (Q × f > 8000 GHz), both dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) dropped. The X‐ray diffraction patterns showed a single phase for all compositions. Typically, the research gained temperature coefficients at resonant frequency around + 10 ppm/°C, while kept high relative permittivity and quality factor value.     

Structural Investigation and Functional Properties of MgxNi1−xFe2O4 Ferrites

The preparation, structural, microstructural, dielectric, and low temperature magnetic properties of Mg_xNi_1?xFe₂O₄ (x = 0, 0.17, 0.34, 0.50, 0.66, 0.83, 1) ferrites synthesized by using a self‐combustion sol–gel method is presented. Good insulating properties were found for all the compositions at high frequencies (kHz and MHz range), which might drive the present ceramics as interesting for RF/microwaves applications. By increasing the Mg²⁺ concentration, the total resistivity strongly increases (from ~10⁶ Ωm for the Ni ferrite to 10⁹ Ωm for the Mg ferrite), corresponding to conductivities in the range 10^?9–10^?6 S/m at f = 1 Hz. Typical ferrimagnetic character with a small coercivity and saturation magnetization in the range (30–50) Am²/kg, which slightly decreases with increasing the Mg content, were found. On the basis of the combined results from the infrared spectroscopy and XRD analysis, it was shown that the magnetic properties depend on the Mg²⁺ ions distribution on the octahedral and tetrahedral sites and the experimental saturation magnetization allowed to compute the cation distribution for the Mg_xNi_1?xFe₂O₄ ferrites.     

Nanocrystalline/Nanosized Ni1−γFe2+γO4 Ferrite Obtained by Contamination with Fe During Milling of NiO–Fe2O3 Mixture. Structural and Magnetic Characterization

The nanocrystalline nickel ferrite (NiFe₂O₄) was synthesized by reactive milling starting from equimolar mixture of oxides. The iron contamination during milling leads to a solid state reaction between Fe and NiFe₂O₄ spinel. This reaction starts for a milling time longer than 30 h. A mixed nickel–iron ferrite (Ni_1?γFe_2+γO₄) and elemental Ni are obtained. The evolution of the nickel–iron mixed ferrite during milling and its properties were investigated using X‐ray diffraction, Fourier Transform Infrared Spectroscopy (FTIR), Laser Particles Size Analyzer and magnetic measurements. Annealing treatment (350°C/4 h in vacuum) is favorable to the reaction between phases. Replacement of Ni²⁺ cations by iron cations provided by contamination leads to the increase of lattice parameter value of the spinel structure. The magnetization of the nickel–iron mixed ferrite newly formed is larger than the nickel ferrite magnetization (13.6 μ_B/f.u. and 6.22 μ_B/f.u., respectively), due to the magnetic moment of Fe²⁺ cation which is double as compared to the Ni²⁺ cation. Magnetization of the milled samples decreases during milling due to the structural changes induced by milling in the nickel–iron mixed ferrite. The annealing induces a reordering of the cations which leads to a larger magnetization.     

Semiconductivity in Acceptor‐Doped BaTi1−xHoxO3−x/2−δ/2

Acceptor‐doped BaTiO₃ powders of formula: BaTi_1?xHo_xO_3?x/2?δ/2: x = 0.0001, 0.001, 0.01, 0.03, and 0.07, were prepared by sol‐gel synthesis, fired at 800°C–1500°C and either quenched or slow‐cooled to room temperature. Electrical properties of ceramics depended on firing conditions, Ho content, and cooling rate. Pellets of all x values fired at 800°C–1000°C were insulating and, from the presence of OH bands in the IR spectra, charge balance appeared to involve co‐doping of Ho³⁺ and H⁺ ions without necessity for oxygen vacancy creation. At higher firing temperatures, OH bands were absent. Pellets fired at 1400°C in air and slow cooled were insulating for both low x (0.0001) and high x (0.07) but at intermediate x (0.001 and 0.01) passed through a resistivity minimum of 20–30 Ω cm at room temperature, attributed to the presence of Ti³⁺ ions; it is suggested that, for these dilute Ho contents, each oxygen vacancy is charge compensated by one Ho³⁺ and one Ti³⁺ ion. At higher x, charge compensation is by Ho³⁺ ions and samples are insulating. A second, more general mechanism to generate Ti³⁺ ions, and a modest level of semiconductivity, involves reversible oxygen loss at high temperatures.     

Almost complete peritectic reaction in YBa2(Cu1−xFex)3O7−δ crystallization involving nanosized primary phase

Study on peritectic reaction is a matter of significant importance in materials science, which generally involves in the solidification of most functional oxide materials, for example, the YBa₂Cu₃O_7?δ (Y123) superconductor could be grown via a reaction of Y₂BaCuO₅ (Y211) + Liquid→Y123. Due to its crystallization characteristic, the growth of those materials does not entirely proceed, which severely impedes the development of industrialized process. Thus the realization of a complete peritectic reaction is an interesting issue for both theory and experiment. Here, we report an almost complete peritectic reaction occurring in the growth of YBa₂(Cu_1?xFe_x)₃O_7?δ crystals using modified melt‐growth. Our findings remarkably show that Y211 almost fully reacted with liquid to generate Y123, remaining approximately 1 vol%, evidently lower than that in the normal case. The nature of this unconventional phenomenon is clarified that the Fe‐doping elevates the nucleation barrier in the peritectic melting of Y123 and causes a massive homogeneous nucleation catastrophe, leading to nanosized and dispersive Y211 particles, which readily and almost fully dissolve in the subsequent peritectic solidification of Y123. Most importantly, the new conception derived from this work is promising for reviving other functional materials, which are disregarded due to their incomplete peritectic reactions.     

Enhanced Luminescence of Ca14Mg2Si8O28+δN4−δ:Eu2+Phosphors by Codoping Ce3+, Mn2+ for White LEDs and Their Energy‐Transfer Mechanism

The Eu²⁺, M‐codoped(M = Ce³⁺, Mn²⁺) phosphor powders were prepared by a solid‐state reaction. The addition of Ce³⁺ in the Eu²⁺ sites in partially nitridated bredigite‐structure phosphor(CMSN) remarkably enhances the luminescent intensity by ~180% through sensitized luminescence. Dual band emission was observed for Eu, Mn‐codoped CMSN through energy transfer from Eu²⁺ to Mn²⁺. Ce³⁺–Eu²⁺ and Eu²⁺–Mn²⁺ energy‐transfer mechanism was investigated through decay profile analysis using Inokuti–Hirayama model and energy‐transfer parameters are determined. Interaction mechanism was identified as dipole–dipole interaction. In addition, phosphor in glass plates was prepared using the phosphor and its feasibility in white LED application was studied and is presented.     

New single‐component multicolor emission Na1−xAl1+2xSi1−2xO4:xBi3+/Eu3+ phosphors via energy transfer

A series of emission‐tunable Na_1?xAl_1+2xSi_1?2xO₄:xBi³⁺/Eu³⁺ phosphors were synthesized via high temperature solid‐state reaction method. The luminescence properties, energy transfer from Bi³⁺ to Eu³⁺ ions, color tuning, thermal stability and quantum efficiency were systematically investigated. Especially, in the host, a certain amount of Si⁴⁺ were replaced by Al³⁺ in order to remedy the charge compensating defect, so that, the emission intensity had been improved. The results of Rietveld refinements, the analysis of SEM mapping and the fourier transform infrared (FT‐IR) indicated that this charge balance strategy was an effective method. Meanwhile, the energy transfer from Bi³⁺ to Eu³⁺ can be inferred and confirmed and the mechanisms were demonstrated to quadrupole–quadrupole interaction. The emission hue can be tuned from blue to pink, and finally to orange red light by properly varying the ratio of Bi³⁺ and Eu³⁺. Importantly, when the temperature was raised to 150°C, the integrated emission intensity was 71.20% of the initial value for NAS:1%Bi³⁺,2%Eu³⁺ samples indicating that these phosphors had excellent thermal stability and stable color (no emission shift). All these properties indicate that the developed phosphors may be potentially used as single‐component color‐tunable‐emitting phosphors for UV light‐emitting diodes.     

Structural and Piezoelectric Properties of (1−x)Pb(Zr1−yTiy)O3–xPb(Zn0.4Ni0.6)1/3Nb2/3O3 Ceramics Near Triple Point

The crystal structure and piezoelectric properties of (1?x)Pb(Zr_1?yTi_y)O₃‐xPb(Zn_0.4Ni_0.6)_1/3Nb_2/3O₃ [(1?x)PZ_1?yT_y‐xPZNN] ceramics were investigated. The 0.665PZ_0.45T_0.55‐0.335PZNN ceramic has the triple point composition, where the rhombohedral, pseudocubic, and tetragonal structures coexist. Maximum d₃₃ and k_p values of 770 pC/N and 0.69, respectively, were observed from this specimen; it also exhibited a large ε^T₃₃/ε_o value of 3250. Although the maximum d₃₃ value was obtained from the triple point composition specimen, its g₃₃ and d₃₃ × g₃₃ values were relatively small because of its large ε^T₃₃/ε_o value. However, the 0.665PZ_0.46T_0.54‐0.335PZNN ceramic, which has a rhombohedral structure, exhibited a large g₃₃ value of 43 × 10^?3 Vm/N and a d₃₃ × g₃₃ value of 27 000 × 10^?15 m²/N. Therefore, this ceramic is a good candidate for multilayer actuators and piezoelectric energy harvesters.     

La1−xCaxCo1−yMgyO3 nano‐perovskites as CO oxidation catalysts: Structural and catalytic properties

In this work, structural and catalytic properties of LaCo_1?yMg_yO₃ and La_1?xCa_xCo_0.50Mg_0.50O₃ nanocatalysts with x=0.00, 0.15, 0.30, 0.45, 0.60, 0.75 and y=0.00, 0.25, 0.50 prepared by citrate method are investigated. The structural characterization using X'Pert package and Fullprof program is an evidence for structural phase transition. The structural results suggest the presence of Co⁴⁺ and Co³⁺ ions that help to overall oxidation activity of these samples and this feature is higher for x=0.60. When substituting Mg and Ca in LaCoO₃ the catalytic activity of CO oxidation shifts to lower temperature. La_0.5Ca_0.50Co_0.5Mg_0.5O₃ nanocatalyst exhibits excellent CO conversion rate of 95% at 267°C. These results suggest that the lower temperature conversion of the sample x=0.60 in series of La_1?xCa_xCo_1?yMg_yO₃ can be mainly attributed to the presence of (i) higher Co³⁺–O–Co⁴⁺ couples, (ii) a structure having the higher tolerance factor, and (iii) a lower crystallite size.     

Magnetoelectric Response in (1−x)PbZr0.65Ti0.35O3–xBaFe12O19 Multiferroic Ceramic Composites

The magnetoelectric (ME) properties of PbZr_0.65Ti_0.35O₃–BaFe₁₂O₁₉ (PZT–BaM) multiferroic ceramic composites have been investigated in this work. These materials, containing both the ferroelectric PZT and ferrimagnetic BaM phases, were prepared from the conventional solid‐state reaction method, and their main (micro)structural characteristics are provided. The ME measurements were performed over wide frequency and temperature ranges, and the influence of the BaM content was appraised. An enhanced ME coefficient (α) was obtained for the applied magnetic field of 0.8 kOe, at 97 kHz. In addition, the results reveal a high thermal stability of α in the whole analyzed temperature range, showing that the studied PZT‐based ceramic composites have great potential for practical applications.     

Electrical transport and thermoelectric properties of Ca0.8Y0.2−xDyxMnO3−δ (0 ≤ x ≤ 0.2)

Ca_0.8Y_0.2?xDy_xMnO_3?δ (0≤x≤0.2) samples were fabricated by the solid‐state reaction method, and their thermoelectric properties were studied from 500°C to 800°C. Upon the substitution of Dy³⁺ for Y³⁺ in the Ca_0.8Y_0.2?xDy_xMnO_3?δ, the electrical and thermal conductivities gradually decreased with increasing Dy³⁺ concentration, whereas the absolute value of the Seebeck coefficient significantly increased. The Ca_0.8Dy_0.2MnO_3?δ showed the largest value of dimensionless figure of merit (0.180) at 800°C as a result of the combination of the largest absolute value of the Seebeck coefficient and the lowest thermal conductivity. We believe that the Ca_0.8Dy_0.2MnO_3?δ is a promising thermoelectric material at high temperatures.     

Proton Conductivity in Solid Solution 0.7(CaWO4)–0.3(La0.99Ca0.01NbO4) and Ca(1−x)LaxW(1−y)TayO4

The conductivity of nominal CaWO₄, CaW_0.99Ta_0.01O_4–δ, 0.7(CaWO₄)–0.3(La_0.99Ca_0.01NbO_4–δ), and Ca_0.9La_0.1WO_4+δ has been studied by means of a.c. impedance measurements. Proton conductivity was observed for CaW_0.99Ta_0.01O_4–δ, which displayed exothermic hydration with enthalpy and entropy of –82 kJ/mol and –120 J/molK, respectively. The proton mobility in CaW_0.99Ta_0.01O_4–δ was low, with enthalpy and preexponential factor of mobility of 82 kJ/mol and 0.7 cm²K/Vs. The high enthalpy of mobility is interpreted to reflect association between the acceptor dopant and protonic defects, whereas the low preexponential factor of mobility may reflect a lower proton concentration than assumed. Rietveld refinement indicated low solubilities of La on Ca‐site and Ta on W‐site. Proton conductivity was also observed in undoped CaWO₄, however, not in Ca_0.9La_0.1WO_4+δ. The conductivity of 0.7(CaWO₄)–0.3(La_0.99Ca_0.01NbO_4–δ) behaved much like that of undoped LaNbO₄, likely due to a very low acceptor dopant concentration.     

Structural elucidation and iron oxidation states in situ formed β‐Ca3(PO4)2/α‐Fe2O3 composites

A detailed structural analysis on the in situ synthesized β‐Ca₃(PO₄)₂/α‐Fe₂O₃ composites is demonstrated. Compositional ratios, the influence and occupancy of iron at the β‐Ca₃(PO₄)₂ lattice, oxidation state of iron in the composites are derived from analytical techniques involving XRD, FT‐IR, Raman, refinement of the powder X‐ray diffraction and X‐ray photoelectron spectroscopy. Iron exists in the Fe³⁺ state throughout the investigated systems and favors its occupancy at the Ca²⁺(5) site of β‐Ca₃(PO₄)₂ until critical limit, and thereafter crystallizes as α‐Fe₂O₃ at ambient conditions. Fe³⁺ occupancy at the β‐Ca₃(PO₄)₂ lattice yields a Ca₉Fe(PO₄)₇ structure that is isostructural with its counterpart. A strong rise in the soft ferromagnetic behavior of β‐Ca₃(PO₄)₂/α‐Fe₂O₃ composites is obvious that depends on the content of α‐Fe₂O₃ in the composites. Overall, the diverse level of iron inclusions at the calcium phosphate system with a Ca/P ratio of 1.5 yields a structurally stable β‐Ca₃(PO₄)₂/α‐Fe₂O₃ composites with assorted compositional ratios.     

Inhomogeneous Structure and Magnetic Properties of Aurivillius Ceramics Bi4Bin−3Ti3Fen−3O3n+3

Structures of Aurivillius ceramics Bi₄Bi_n ? 3Ti₃Fe_n ? 3O_3n+3 with n = 3–9 have been investigated by X‐ray powder diffraction and high‐resolution transmission electron microscopy. The Aurivillius ceramics Bi₄Bi_n?3Ti₃Fe_n?3O_3n+3 with n = 3–5 exhibit homogeneous layered structure. However, the ceramic samples with n = 6–9 are composed of mixed Aurivillius phases with different layer numbers, in which the stacking faults of the disordered intergrowth of different n‐layered perovskite blocks and the so‐called “side‐stepping” of the (Bi₂O₂)²⁺ layers are generally observed. Magnetic measurements indicate that Fe cations couple antiferromagnetically and the inhomogeneity of composition exists in the samples. The magnetic properties are obviously influenced by the inhomogeneous structure and composition.     

Structure Evolution and Electrical Properties of Y3+‐Doped Ba1−xCaxZr0.07Ti0.93O3 Ceramics

Lead free piezoelectric ceramics of Y³⁺‐doped Ba_1?xCa_xZr_0.07Ti_0.93O₃ with x = 0.05, 0.10, and 0.15 were prepared. Composition and temperature‐dependent structural phase evolution and electrical properties of as‐prepared ceramics were studied systematically by X‐ray diffraction, Raman spectroscopy, impedance analyzer, ferroelectric test system, and unipolar strain measurement. Composition with x = 0.10 performs a good piezoelectric constant d₃₃ of 363 pC/N, coercive field E_c of 257 V/mm, remanent polarization P_r of 14.5 μC/cm², and a Curie temperature T_m of 109°C. High‐resolution X‐ray diffraction was introduced to indicate presence of orthorhombic phase. Converse piezoelectric constant d₃₃* of x = 0.10 composition performed better temperature stability in the range from 50°C to 110°C. That means decreasing orthorhombic–tetragonal phase transition temperature could be an effective way to enlarge its operating temperature range.     

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides La_xSr_3?xFe₂O_7?δ (LSFO7‐x) (x = 0, 0.25, 0.5) was measured by the high‐temperature gravimetry and the coulometric titration. In the composition series, the P(O₂) dependencies exhibited typical plateaus at δ = (2?[])/2. Meanwhile, La_0.5Sr_2.5Fe₂O_7?δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O₂), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr‐site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen‐deficient and the lower decomposition P(O₂). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, , and the partial molar entropy of oxygen, , calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.